The differentiation of organs proceeds from the differentiation of their constituent cell populations. In each cell lineages, cytodifferentiation can be viewed as a succession of phases which include primary cell sorting and organogenesis, cell proliferation, cell specialization and peak differentiation. Each phase is delimited by a discrete transition points. The project described herein aims to use an insect exocrine gland as a model in which to chronicle the phenomenology of developmental transitions, to evaluate the hormonal stimuli which are required for such transitions, and to evaluate the intracellular intermediate events which lie between commitment and a change in phenotype with a view to discovering which of these intracellular-events are rate-limiting in the ongoing process of cell differentiation. Differentiation of the spermathecal accessory gland (SAG) of the female mealworm beetles begins in the last instar larva, continues in the pupa, and culminates at one week after ecdyses to the adult. The total period is about three weeks. Within this time span, the sternal epithelium invaginates and undergoes morphonegetic movements to yield a stubby sac of columnar epithelium at the time of ecdysis to the pupa. Within the pupa, the cells divide rapidly and diverge into two cell populations, one (cuticulogenic) which undergoes terminal differentiation during deposition of the cuticular export system while the other (secretory) undergoes terminal differentiation only after ecdysis to the adults. The extent of differentiation can be scored on both morphological and biochemical criteria, and it occurs within fairly-defined endocrine contexts. We have already described many features of the cytodifferentiation in the SAG during the pupal and adult stages. In the proposed study, we intend to refine certain aspects of this description and to extend it backward into the prepupa. By ablation of endocrine centers, by culture of rudiments in vivo and in vitro, and by replacement therapy, we will determine when developmental hormones are necessary to support the ongoing cytodifferentiation. By administration of precursors, extraction and fractionation of nucleic acids, and analysis of their character and time of synthesis, we hope to pinpoint the prerequisites for transitions and the effects of hormone upon the synthesis, stability, and utilization of informational macromolecules.